Stress indicating bolt or stud



June 10, 1952 A. R. STONE 2,600,029

STRESS INDICATING BOLT OR s'run Filed Nov. 12, 1946 TEE-:1 f 25 W |m+ I22 24 23' /2o INVENTOR ALBERT RIVINGTON STONE.

HIS AT ORNEY Patented June 10, 1952 STRESS INDICATING BOLT OR STUD Albert Rivington Stone, Baltimore, Md., assignor to Gerotor May Corporation, a corporation of Maryland Application November 12, 1946, Serial No. 709,203

6 Claims.

My invention is directed to a new form of stressindicating stud or bolt, and more particularly concerns a method and means for determining rapidly and with a high degree of sensitivity the exact degree of tightness, and of stresses and strains imposed thereon.

My invention has for an objective, the production of a stress-indicating stud, bolt or other fastening means which is simple, sturdy, reliable and efiicient in operation, combining therewith the essential elements of low first cost and ease of production.

Another object is to produc a stress-indicating stud or bolt assembly employing a stud or bolt or other fastening device of the type generally described just hereinbefore, and combining therewith simple, efiicient, reliable and inexpensive means for indicating the stressing of the bolt with a minimum of expenditure of time, effort and operational expense.

A further object is to produce a method and mode of operation for achieving the aforementioned objectives in simple, efficacious manner.

Other objects and advantages will in part be obvious and in part more fully pointed out hereinafter during the course of the following specification taken in the light of the accompanying drawings.

My invention accordingly resides in the several operational steps, parts, elements and features of construction, combination of parts, and in the combination of each of the same with one or more of the others, the scope of the application of all of which is more fully set forth in the claims at the end of this specification.

In the drawings, wherein I disclose several embodiments of my invention which I prefer at present,

Figure 1 discloses, partly in section and partly in schematic disclosure, one embodiment wherein a single electrical terminal is employed, the conducting wire being fast thereto at the outer end of the bolt, stud or other fastening device, and the bolt or the like, or the wire serving indiscriminately as the other terminal of the electrical circuit; while,

Figure 2 is a view similar to Figure 1, wherein conducting wires through the bolt or stud are at all points and all times insulated fully from the bolt or stud, and paired terminals are employed for conducting the electric current to and from the gauge wires.

Throughout the several views of the drawings like reference characters indicate like parts.

As conducive to a more thorough understanding of my invention it may be noted at this point that this application is in many respects a companion to my pending application for Letters Patent of the United States, Serial No. 588,650, filed April 16, 1945, entitled Fastening Device,

now Patent No. 2,413,797. Therein I have disclosed a bolt or stud which can be tightened down to a predetermined stress or elongation or both. Additionally, I disclose a method of achieving this objective. Essentially, that construction and technique of operation contemplates providing an elongated longitudinal bore within the bolt or stud. This bore may extend entirely through the bolt or other fastening device or may terminate short of the end thereof. Within the bore I provide a gauge pin which remains continuously unstressed. In one embodiment when the bore is unstressed the design is such that the free end of the gauge pin is exactly flush with the outer face of the head of the stud or the like. Upon exact tightening down of the bolt, however, it is elongated to a certain extent, whereas the unstressed gauge pin remains at its original length. The degree of separation between the end of the unstressed pin and the outer face of the stressed bolt constitutes a precise measure of the stress within the bolt or of length of bolt grip, or both.

While in my said companion application I disclose what constitutes a distinct advance in the art, namely, a stress-indicating stud, bolt or other fastening device, the latter is entirely mechanical in both its concept and its operation. An important limitation of such construction is that it provides for indication, either transitory or recorded, for only one particular setting of snap means responding to a given external stress. In many instances, however, it is obviously highly desirable to provide for continuous reading over a wide rang of stressing. Thus, for example, when seating a bolt or other fastening device equipped according to the teachings of my present invention, the operator at all times is aware of the degree of stressing and the approach to the predetermined optimum value. Similarly, when in use variations in stresses, tending towards incipient failure, can be instantaneously determined.

Moreover, some difliculty in the use of oldstrain gauges is encountered in that the gauge reading heretofore could be determined only locally, in the vicinity of the particular fastening devices undergoing observation. Frequently, however, this will be located in spots so remote as to be difficult if not even impossible of access. It is highly desirably that the stress reading can be determined at some readily accessible point, which necessarily must often be remote from the particular fastening device. In a thoroughly practical installation according to my invention, the meters or other indicating devices will be located in a single bank, readily accessible to a single operator.

7 My invention accordingly may be considered as avoiding in substantial measure the several 3 limitations of the prior art and to provide for the first time for continuous stress readings over a wide range thereof, and these at points remote from the fastening means themselves, and readily accessible to the operator.

A further object is to provide an assembly of the general type described, in which any form of electronic indicator or recording system may be employed.

Generally, my invention may be considered as comprising a conventional strain gauge disposed inten'orly of a suitable fastening device, the assembly being employed in electrically controlling the tightening means or for indicating stresses throughout the tightening and subsequent use. Combined with it is a remotely located indicator of the stressin occurring within the fastening device. Any sort of electronic indicator or recording system may be employed, such for example as the instruments employed in the conventional strain gauge. In carrying my invention into execution, I employ an ordinary bolt, stud, or other fastening device, constructed of metal. This bolt is longitudinally bored. Preferably, the

bore is centrally disposed. In the preferred embodiment this here extends the entire length of the bolt or stud but conceivably may terminate short of the threaded end thereof. In each embodiment one or more fine resistance wires extend through said bore. cording to Figure l the wire is made fast at the end of the bore adjacent the threaded shank of the bolt, while its other end, extending to the outer face of the head of the belt or stud, passes through an insulating block recessed in the head of the bolt. This end of the wire is received in one terminal of an electric circuit, later to be described.

It will be seen that unlike the gauge rod of my companion application, which is free at the head end of the bolt, the gauge wire, that is, the electric resistance wire here employed, is fast at both ends of the bore and is stressed along with the stressing of the bolt. Reliance is upon the electrical phenomenon that stressing of the wire, bringing about change in the cross-sectional diameter thereof, is accompanied by corresponding change in its electric resistance. Upon preliminarily properly calibrating this wire, a measure of the stressing of the bolt or stud can be achieved by passing an electric current therethrough in a simple series circuit and by measuring the variation in the resistance of the wire. The decrease of current flow serves as ameasure of the stressing of the bolt and proper observations can be made in rapid and simple manner.

In the embodiment according to Figure 2, two resistance wires are led through the bore provided in the bolt or nut and are insulated at each end of the bore and throughout its length from the walls defining the bore. Resistance blocks are provided for receiving the wire. Paired terminals are provided in the insulating block recessed in the head of the bolt for connecting the resistance wires to the electric circuit. Otherwise, the assembly is substantially that hereto fore described with respect to the embodiment of Figure 1.

And now having more specific reference to the construction according to Figure 1 it will be seen that the conventional bolt, stud or other fastening device, indicated generally at H), comprises an elongated shank having a threaded portion l2. indicated generally at 13.

In the embodiment ac- 4 The usual enlarged head is provided,

The bolt is conveniently rounded at its threaded end as indicated at M. I provide a longitudinal bore I5 within both the head and shank portion of the bolt. While it is entirely possible to have this bore disposed asymmetrically with reference to the axis of the bolt, convenience of construction dictates that this latter should be disposed centrally thereof, as is the case in the embodiment undergoing description. This bore illustratively extends entirely through the length of the bolt. It is entirely within the scope and province of my invention, however, that this bore, opening to the exterior at the outer surface of the head portion [3, terminates short of the shank end of the bolt.

I provide within the bore |5 a resistance wire it; of small diameter, having substantial resistivity, and conveniently formed, for example, of nickel-chrome or other similar alloys possessing comparatively high resistance characteristics. This wire is made fast to the bolt ID or the like at the shank end of the bore I5. In the embodiment illustrated, this is the extreme left end of Figure l, at the shank end thereof. The wire l5 may be considered to be shorted at one end thereof to the bolt I0.

Otherwise, this wire is insulated from bore l5 throughout its extent therethrough. At the head end |3 of the bolt I0 I provide fast thereto a terminal H for the wire I6. To insure that this terminal I1 is insulated from the metal head I3, I provide suitable insulating means, here disclosed as an insulating bushing or insert l8, provided in a corresponding recess |9 formed in the head of the bolt.

It will be seen from the foregoing that unlike the unstressed pin of the construction according to my co-pending application, the wire I6 is fast at both ends to the bolt I0. It is stressed along therewith. Advantage is taken of the phenomenon that upon elongation of a metal filament, a corresponding decrease in cross-sectional dimensions occurs, and upon the further phenomenon, electrical in nature, that upon decrease in crosssectionalarea of an electric conductor, increase occurs in the ohmic resistance thereof. Upon passing an electric current through this resistor, and upon suitably calibrating the wire to determine increase in resistance per unit change in stress, it is possible, through the use of suitable meters, to determine precisely the elongation and stressing of the bolt.

To this end, I provide an energizing circuit in series with said filamentary Wire l5. This circuit comprises a lead 20 extending from the terminal I! to a source of electric supply either alternating current or direct current, here conveniently shown as a battery 2|. A variable resistance 22 is conveniently interposed for nicety of adjustment to insure greatest current sensitivity. This may be omitted, however, if found desirable. A current lead 23 extends from the shank end of the bolt ID to the ohmic resistance 22. This lead 23 may be connected directly to the bolt ID or to one terminal of the wire |5. A conductor 24 extends between the resistance 22 and the source of current supply 2|. Finally, a meter, here shown as ammeter 25, is interposed in the lead 20. This accurately indicates changes in current response upon variation in resistance in the wire l6.

For clarity, the circuit through the filamentary wire l6 may be traced from the positive side of the battery 2|, through lead 20 to ammeter 25, and thence to terminal I1, thence across wire it to bolt ID, to lead 23, to resistance 22, and back in series, circuit across lead 24 tothe negative pole of the battery 2|.

In use, the bolt, after suitable calibration of the wire I 6, is threaded down into the workpiece. Elongation occurs in the wire I6, as an incident to such procedure. When such elongation occurs, reduction is brought about in the cross-sectional diameter of the wire, so that resistance build-up is encountered. This increase in resistance, given constant voltage source of current supply 2|, is attended by a decrease in the current flow therethrough. This change in current flow is evidenced upon meter 25. By proper calibration, the variation in current carried is a measure of elongation of the bolt or stud. It is sufficient for all practical purposes that the meter be calibrated in terms of a particular wire size, the small variations within any particular roll of wire from which the filament I6 is selected being found to be insufficient in practice to set up appreciable disturbance in the experimental data. Variable resistance 22 is introduced simply to permit the current to be adjusted preliminarily for a given diameter of wire Hi, the objective being to bring the initial reading of the ammeter 25 near one end of the dial.

The embodiment of Figure 2 is substantially in accord with that of Figure 1, with a certain fundamental exception hereinafter to be pointed out, the external circuit being exactly similar with that of the first embodiment. For this reason, and since further description of such external circuit would constitute mere surplusage, no

such description is included at this point.

The fundamental distinction between the disclosure of Figure 1 and that of Figure 2 is that while in Figure 1 a single filamentary wire l6 has been disclosed, made fast at one end within the bore and at its other end to the terminal IT, in the embodiment according to Figure 2, two wires [5A are employed. They are insulated from the walls of the bore I5, that is, from the bolt l0, through the entire extent of these wires. At one end of the bore [5, here shown as the end M of the shank of the bolt, 9. second insulating plate or the like 26 is provided, conveniently sunk into a recess 21 provided in the end of the shank portion. This plate 26, made fast to the end of the bolt, receives the ends of the wires 16A and joins them together. The wires are then strung through the bore [5 into the plate [8 at the head end of the bolt similar to the plate 18 hereinbefore described with respect to the construction according to Figure 1. The free ends of the wires IGA are joined to terminals 28 and 29 in the plate I8 by which terminals the wires 16A are joined to leads 23, 20 respectively. The closed circuit for the electric current will be obvious.

The necessity is avoided of nicety of gauging the position of the end of the gauge pin relative to the end of the bolt head. All that is necessary is to secure the electric leads into position, adjust for zero load rating, and then tighten the bolt, stud or other fastening device so as to elongate the same to the specified reading on the gauge 25. A high degree of rapidity is insured with accurately reproducible results and this with the use of but a minimum of labor, of no particular skill. All these and many other high advantageous results attend upon the practice of my invention.

Since many modifications of the present embodiments of my invention will readily occur to those skilled in the art, once the details thereof are disclosed; and since similarly, many embodiments will suggest themselves, falling within the sphere of my present contribution to the art, I desire that the foregoing disclosure be considered as simply illustrative and not by Way of limitation.

I claim:

1. An elongated, longitudinally-bored stud or bolt, a block of insulating material fixedly secured to said bolt or stud at each end of said bore, and a plurality of fine wires extending through said bore and carried between said blocks and being insulated from the Walls of the bore, the terminals of said wire or wires being electrically coupled at one insulating block and ending in terminals at the other block.

2. An elongated, longitudinally-bored bolt or stud having an insulating block fixedly received in one end of said bore, and an electric currentcarrying wire extending through said bore and secured at one end thereof to said bolt and at its other end to said insulating block.

3. In combination, an elongated, longitudinally-bored bolt or stud, an insulating block fixedly secured to the bolt or stud at one end of said bore, an e1ectrical-current-carrying wire extending through said bore and secured to the bolt at both ends thereof, at least one said end being secured by way of said insulating block, and an electric circuit connected electrically to theends of said wire and including a source of current supply and an indicating meter.

4. In combination, an elongated, longitudinally-bored bolt or stud; an insulating block provided therein at at least one end of said bore and fixedly secured therein; an electrical currentcarrying wire extending through said bore and secured to the bolt at both ends thereof, at least one said end being secured thereto by way of said insulating block; and an electrical circuit connected electrically to the ends of said wire including a variable ohmic resistance, a source of current supply and an indicating meter.

5. In combination, an elongated, longitudinally-bored bolt or stud having a block of insulating material fixedly secured to said bolt or stud at each end of said bore, electrical current-carrying wires extending through said bore and suspended from said insulating blocks, said wires being electrically coupled at one said insulating block and ending in terminals at the other insulating block, and an electric circuit connected with said terminals and including a source of electrical supply and an ammeter.

6. As an article of manufacture, an elongated longitudinally-bored stud or bolt, an electrical current-carrying wire extending through said bore, means insulating said wire from the bore of said bolt or stud, and insulating means securing said wire to the bolt or stud at each end of said bore.

ALBERT RIVINGTON STONE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,036,458 Carlson Apr. 7, 1936 2,292,549 Simmons Aug. 11, 1942 2,298,216 Lamberger Oct. 6, 1942 2,405,199 Faust Aug. 6, 1946 FOREIGN PATENTS Number Country Date 397,701 Great Britain Aug. 31, 1933 

